Completely safe function that discards the constraint type
(which is a phantom type). You can "remember" it again by
using unsafeRemember from the Unsafe module.

Constraint types

There are currently nine types of constraints. When
appending a number to a property, we hint that it refer to
one of the bodies that the constraint is intercting with
(e.g. "Second anchor" is the position of the anchor on the
second body in its coordinates).